The MK3 head is supposed to be good for a 25bhp bump, but again caution is needed. Most of the bump is from the exhaust manifold which is a split 3/3 pipe system that flows miles better than the hockey stick rubbish fitted as stock. Unfortunately, most say that too would foul the steering box, so beware.
Gary

Same situation with my Zodiac, father bought a Mark 2 head instead of a Mark 1, my father is checking with some guys in thr U.K. for a new exhaust manifold, the hockey stick is very restrictive.

Taking the Zephyr home! We're currently on the road back towards Chicago. Will get her cleaned up and more pics coming early this week.

I see you are missing the rear bumper, there is this company in Vietnam that makes new Mark 2 bumpers, they are quite nice. They are thinner than the originals, since they are made out of Stainless Steel, but they have the look! You can find them on eBay.